Airfoils of the type forming the blades and vanes of gas turbine engines are manufactured to rigidly designed specifications. Accurate geometric contour and size are essential to achieving aerodynamically efficient operation. Should an airfoil deviate from its design contour and chord length, adverse aerodynamic impact is likely to extend downstream into successive blade and vane stages.
Blades and vanes are among the most expensive of engine components to manufacture and among the most susceptible to wear. Blades and vanes within the turbine section are particularly susceptible to thermally induced damage, wear and abrasion. It is, therefore, that blades and vanes are commonly repaired or remanufactured after use to restore original dimensional characteristics to the engine without the expense of totally new components.
Blades and vanes for repair or remanufacture are carefully selected in a screening process to determine those components which are not so severely damaged that repair would be uneconomical. One critical measurement in the screening of airfoils is the airfoil chord length, the straight line distance between leading and trailing edges of the blade.
Many gauges are known in the art to be capable of measuring chord length. The accuracy and speed at which such gauges measure chord length varies. In general, known gauges such as those illustrated in U.S. Pat. No. 3,639,992 to Dabrush entitled "Chord Length Gauge" are suitable for use, but may have limitations where a straight line reference on the blade or vane is not available. The contour of a usual reference line, commonly the leading or trailing edge, may not have been a straight line as manufactured or may have become distorted in the hostile, high temperature engine environment.
Engine manufacturers and airfoil repairers, therefore, seek yet improved methods and apparatus for measuring chord length notwithstanding deviations of the measurement reference points from a straight line.